116 



MALACOZOA. GASTEROPODA. FULMOBRANCHIATA. 



organs of touch, nor to be nearly so sensible as those of snails. 

 The eyes, small globose, black dots, are placed at the base of 

 the tentacula internally, on the middle of their dilated part, 

 which is of a reddish colour, the long taper portion being dusky 

 brown. The tentacula have no auricle at the base, a character 

 given in most books as generic, which it therefore is not. The 

 orifices of the pulmonary, alimentary, and genital organs are 

 on the left side, but these organs themselves on the right. 

 When the foot and head are protruded, a large portion of the 

 last spiral turn is filled with air, so that probably the shell is 

 very little heavier than water. Out of the water, the animal 

 in advancing drags the shell flat, or lying in the natural position, 

 with the right side of the disk to the right, and it cannot ad- 

 vance in any other position. These observations are quite 

 sufficient to prove that this species is truly dextral, and that 

 the concave surface of the disk is the upper. 



Planorbis vortex resides, not in the mud, but on its surface, 

 and more especially on the stems and leaves of plants, generally 

 in the water, but often also out of it ; and it retires to rest into 

 cavities or recesses on the banks, formed by overlying stems 

 and leaves, or upon the plants themselves. Individuals fre- 

 quently crowd together to repose. In long-continued droughts, 

 it closes the aperture of the shell with a whitish membranous 

 substance, like that of the Helices. 



Planorbis vortex is liable to considerable distortion, some of 

 the turns frequently projecting beyond their proper level, and 

 sometimes the whole disk preter-naturally hollowed out on one 

 side, and protruding on the other. 



First found by me, on the 20th of July, 1 842, on aquatic 

 plants, and on the mud, in a ditch with stagnant and rather 

 putrid water, in the hollow between Aberdeen and the Spital, 

 where it is abundant. 



Planorbis vortex. Muller, Verm. Terrestr. et Aquat. ii. 158. 

 Planorbis vortex. Var. a. Drap. Moll. 45. PI. 2. f. 4, 5. Helix 

 spirorbis. Mont. Test. Brit, 455 ; Suppl. PI. 25. f. 2. Planorbis 

 vortex. Larak. Syst. vi. 2. 154. Ed. 2. viii. 385. Planorbis spiror- 

 bis. Gray's Turton, 267. PL 8. f. 91. Planorbis vortex. Flem. 

 Brit. Anim. 277. 



A. Planorbis Cortex crdssulus. Thicldsh Flat Coil-Shell. 



Shell extremely depressed, very thin, pellucid, shining, 

 regularly and equally concave above and beneath ; with five 

 gradually increasing volutions, which are convex above and 

 beneath, and transversely obsoletely striulate, with scarcely 



